Some elements constituting a personal wireless network as illustrated in FIG. 1 may be considered. The most basic element is a station. Here, FIG. 1 is a view illustrating connection relationships between stations in a general personal wireless network system.
In general, a piconet is established when two or more stations operating in the same radio frequency channel exist within a personal activity area.
Stations are classified as a master M and slaves S1, S2, S3, and S4 depending on roles thereof.
The master M manages the entirety of the piconet and only one master may exist in the piconet. The master M controls the slaves S1, S2, S3, and S4 by broadcasting beacons to the plurality of slaves S1, S2, S3, and S4.
The slaves S1, S2, S3, and S4 may transmit and receive data under the management of the master M. In general, the master M internally has information of the slaves S1, S2, S3, and S4, and thus, when the slaves S1, S2, S3, and S4 are connected to the master M, the master M performs communication using the information of the slaves S1, S2, S3, and S4. Here, a structure of a superframe transmitted and received between the master M and the slaves S1, S2, S3, and S4 is as illustrated in FIG. 2.
As illustrated in FIG. 2, the structure of a superframe includes a beacon period, a contention period, and a data allocation period, and length so the periods are variable.
During the beacon period, the master transmits a beacon packet including network reference information to the slaves.
During the contention period, the slaves and the master transmit and receive a command packet such as a network join request/separation request/permission, a resource allocation request/permission, a connection request/permission, and the like, in a random access manner. Here, during the contention period, exclusive access to a medium through an exclusive allocation by the master is not guaranteed, and thus, each station (the master and the slaves) access a medium by using a carrier sense multiple access/collision avoidance (CSMA/CA).
During the data allocation period, each station exclusively accesses a medium during a time slot distributed to each station. The master distributes a time slot of the data allocation period to each station by using a time division multiple access (TDMA) scheme. Here, each station may exclusively access the medium during the distributed time slot, and each station may transmit and receive data to and from other station in a one-to-one correspondence manner, without intervention of the master, during the allocated time slot.
FIG. 2 illustrates a structure of a superframe of data allocation periods with respect to slaves S1 to S4 when a data allocation period of the master is M and the overall number of the slaves is 4 (S1 to S4).
In the wireless network system, when the master M and the slaves S1 to S4 perform communication, communication may be interrupted for a long period of time for various reasons such as radio interference between the master M and the slaves S1 to S4, propagation cutoff due to an obstacle, defective hardware, and the like, in many cases.
FIG. 3 is a view illustrating an example in which a radio link between the master station M and the slave stations S1, S2, S3, and S4 is disconnected in a personal wireless network system.
In a case in which the wireless network is disconnected as illustrated in FIG. 3, the master M may consider that there is no data for the slaves S1, S2, S3, and S4 to transmit to the master M so slave data is not received, and thus, the master M may not discriminate whether there is a problem with a link physically or whether the slaves S1, S2, S3, and S4 have no data to be transmitted for a long period of time.
Similarly, the slaves S1, S2, S3, and S4 receive beacons from the master M so the slaves S1, S2, S3, and S4 are synchronized with the master M, but, here, beacons are received even when data is not properly transmitted and received, and thus, the slaves S1, S2, S3, and S4 continue to request transmission from the master M.
Thus, even though there is a problem with a link physically between the master M and the slaves S1, S2, S3, and S4, the master M continues to wait to receive beacon signals from the slaves S1, S2, S3, and S4, and in addition, when the number of slaves increases in a state in which the master M stores information of the slaves S1, S2, S3, and S4 which have already been interrupted in communication, other slaves may lose an opportunity for accessing a network, increasing a waste of memory resource.
Also, the slaves S1, S2, S3, and S4 are supposed to be synchronized with the master M through reception of beacons to transmit and receive data, and here, if data transmission is not properly performed, the slaves S1, S2, S3, and S4 need to recognize the corresponding situation quickly and overcome the limitation or need to search for any other appropriate master nearby to access the network.